Good suggestions here - but are you wanting to make money A-Spec or B-Spec?
Be-Spec takes a lot less effort - your stable of drivers do the work for you, and there are plenty of lucrative B-Spec races, if you have trained your AI well. My AI can take on the game's AI, with 50 PP less themselves and are incredibly aggressive, fiercely determined to win, and never seem to tire.
A-Spec grinding is a different cup of tea - there it is not only making the money, but enjoying the various races you can engineer for yourself, as a Race Director, and a huge variety of cars to try out - while winning and making money, too.
Fun game, isn't it? All you need is a little imagination, and the money piles up faster than you can spend it. I only went Online recently and already reached my 20 Mil ceiling several times (and was also ripped off for several millions that way.)
Making money in GT5 online is easier than making money in GT4. Tried and tested.